Conventionally, the ink jet printing device is used in the image forming apparatus, such as copier, printer, facsimile, multi-function peripheral, etc. In this ink jet printing device, the head unit includes a plurality of recording heads provided with a plurality of discharge nozzles arranged in the sub-scanning direction, and filled up with the inks of the different colors, and high-density and high-quality printing is carried out at high speed by discharging of ink drops from the discharge nozzles of the recording head to the recording sheet according to the image signal while moving the head unit in the main scanning direction.
On the other hand, when printing on the recording sheet with such ink jet printing device, it is known that the dot density (resolution) is determined according to the physical array interval (pitch) of the discharge nozzles.
For example, as shown in FIG. 12, assuming that the pitch of the discharge nozzles 1a, which are arranged at equal intervals on each of the recording heads 1B, 1C, 1M, and 1Y which respectively discharge the inks of the black (B), the cyan (C), the magenta (M) and the yellow (Y), and constitute the head unit 1, is set to H, and that the pitch H is equal to 1/150 inches, the dot density according to the pitch of the discharge nozzles 1a is determined as being 150 dpi.
When the head unit 1 performs color printing at the resolution that is the same as the resolution (150 dpi) determined according to the array of the discharge nozzles 1a as shown in FIG. 13A and FIG. 13B, the head unit 1 is moved only in the main scanning direction and the discharging of the ink is carried out sequentially from the recording heads 1B, 1C, 1M, and 1Y, to the recording sheet P. In this case, the ink drops will overlap in order of B, C, M, and Y, and the color image will be printed.
In this case, if the ink discharging is performed while moving the head unit 1 in the forward main scanning direction only, the stacking order of the ink drops of the different colors will be fixed, and the printing result will become uniform.
In contrast to the above case, if the discharging of the ink is carried out while the head unit 1 is moved in the forward main scanning direction and then in the backward main scanning direction as shown in FIG. 14A and FIG. 14B, the stacking order of the ink drops for the backward scanning is reversed to that for the forward scanning. Namely, in the forward scanning, the ink drops overlap in order of Y, M, C, and B, but, in the backward scanning, the ink drops overlap in order of B, C, M, and Y. In this case, color inconsistency will occur in the printing result, and the quality of the printed image will be deteriorated.
Moreover, Japanese Published Patent Application No. 03-056186 discloses an ink jet printing device which carries out printing at a resolution higher than the resolution determined according to the array interval of the discharge nozzles 1a. 
As shown in FIG. 15A, FIG. 15B and FIG. 16, in the ink jet printing device disclosed in Japanese Published Patent Application No. 03-056186, the high-resolution printing is achieved in the following manner. After the recording sheet P is moved to the previous printing region of the recording sheet P where the printing is performed by the main-scanning-direction movement (the first scan) of the head unit 1, in the sub-scanning direction only for the pitch H/2 which is half the pitch of the discharge nozzles 1a, the head unit 1 is moved in the main scanning direction so that the ink drops reach the non-printed region between the discharge nozzles 1a in the sub-scanning direction of the first scan.
In the example of FIG. 15A and FIG. 16, the printing of the two colors of the cyan and the magenta is illustrated for the sake of simplified explanation.
Subsequently, the printing is carried out all over the recording sheet P by moving the head unit 1 in the sub-scanning direction so that the head unit 1 is located to the non-printed region continuous to the printing region, and repeating the printing procedure.
In this case, the printing at the resolution of 300 dpi which is twice the resolution of 150 dpi is realized by moving the head unit 1 in the sub-scanning direction by ½ of the pitch H of the discharge nozzles 1a. Moreover, in order to realize the printing at the higher resolutions which are three times, four times, . . . higher than the resolution of 150 dpi, it is necessary to move the head unit 1 in the sub-scanning direction by ⅓, ¼, . . . of the pitch H of the discharge nozzles 1a. 
Moreover, in the case of printing at the high resolution higher than the resolution which is determined according to the pitch of the discharge nozzles 1a in the above-mentioned way, the ink drops of cyan C reach the recording sheet P when the head unit 1 is scanned in the 1st time in the main scanning direction, the ink drops of magenta M reach the target locations of the recording sheet P between cyan C and cyan C when the head unit 1 is scanned in the 2nd time in the main scanning direction. Namely, the ink drops of cyan C and magenta M alternately reach the target locations on the recording sheet P, and it is possible to prevent the mutual color inconsistency on a scanning line basis on the recording sheet P from being conspicuous.
However, when the ink discharging method of Japanese Published Patent Application No. 03-056186 mentioned above is used, the ink drops of magenta M are printed in the 2nd time to the target locations between cyan C and cyan C the ink drops of which are printed in the 1st time by the head unit 1, and after the recording sheet P is moved in the sub-scanning direction greatly and the head unit 1 is moved in the main scanning direction to the non-printed region on the recording sheet P, the impact sequence of the ink drops will be reversed.
More specifically, as shown in FIG. 17A and FIG. 17B, after the head unit 1 is scanned in the 2nd time in the main scanning direction and the discharging of the ink is completed, the head unit 1 is scanned in the 3rd time in the main scanning direction and the ink is discharged to the recording sheet P. At this time, the impact timing of the ink drops of magenta M (indicated by M1 in FIG. 17B) which reached the target locations from the discharge nozzles 1a of the rear-end portion of the head unit 1 when the head unit 1 is scanned in the 2nd time is earlier than the impact timing of the ink drops of cyan C (indicated by C1 in FIG. 17B) which reached the target locations from the discharge nozzles 1a of the front-end portion of the head unit 1 when the head unit 1 is scanned in the 3rd time.
The reversing of the impact sequence of the ink drops mentioned above arises at the transitional locations between the discharge nozzles 1a of the rear-end portion of the head unit 1 and the discharge nozzles 1a of the front-end portion of the head unit 1 when the recording sheet P is moved in the sub-scanning direction greatly and then the head unit 1 is moved to the non-printed region on the recording sheet P, and consequently the problem that color inconsistency becomes conspicuous on the recording sheet P as a whole will arise.